Direct current motor drive

ABSTRACT

A system in which a DC motor maintains a constant preset speed under changes of load, the preset speed being variable at will. The system includes a constant-current supply, with diverter means to divert varying amounts of current from the motor circuit depending on the motor speed in comparison with a speed setting.

5/1971 Hermansson 318/302 I United States Patent 1 1 1111 3,792,330

Ottoson 1 1 Feb. 12, 1974 [54] DIRECT CURRENT MOTOR DRIVE 3,286,15111/1966 Ding er 318/331 3,422,332 1/1969 Dinger 318/331 [75] mvemorAllen 011050", westboro Mass- 3,514,682 2/1967 Corey 318/369 73Assigneez Vee Arc corpuration, westboro 3,603,857 9/1971 Crane 318/302Mass.

Primary Examiner-Bernard A. Gilheany [22] Ffled' Sept 1972 AssistantExaminer-Thomas Langer [21'] Appl. No.: 288,900 Attorney, Agent, orFirml(enway & Jenney {52] US. Cl 318/269, 318/345, 318/375 [57] ABSTRACT.[51] Int. Cl. H02p 5/16 58 Field 61 Search 318/269, 326-328, A Systemwh'ch a DC mamams a cnstam 8/302 331 345 369 375 preset speed underchanges of load, the preset speed being variable at will. The systemincludes a constant- [56] References Cited current supply, with divertermeans to divert varying 1 amounts of current from the motor circuitdepending UNITED STATES PATENTS on the motor speed in comparison with aspeed set- 3,369,167 2/1968 Hanchett 318/331 ting 3,229,182 1/1966Kubler 318/331 3,577,055 7 Claims, 11 Drawing Figures FROM I49PATENTEDFEBIZIW 3.792.330

SHEET 1 BF 3 FROM I49 FROM I42 3 g FROM I47 g? /54 26 44 EFERENCE BRIDGERVOLTAGE DVERTER RECTIFIER A URE AND SPEED SETTING /53 L FEEDBACKTACHOMETER msmenrimw 3.792.330

SHEET 3 0F 3 FIG. 9

DIRECT CURRENT MOTOR DRIVE BACKGROUND OF THE INVENTION DC motors drivenfrom rectified AC sources through thyristors (silicon controlledrectifiers) have presented difficulties in variable-speed operationbecause of discontinuities in current supply to the motor. A measure ofthe effect of discontinuities in the armature current is afforded by theform factor which is the ratio of rms to average current. A constantdirect current has a form factor of unity and it is desirable toapproach this low value as closely as possible. Only the average currentgoes into producing torque, while the heating increases as the square ofthe form factor.

A high form factor also results in poor commutation. Especially at lowspeeds discontinuities in torque may result in cogging, which isdetrimental to many precision machining operations.

SUMMARY OF THE INVENTION According to the invention, the source ofcurrent for the motor is a constant-current source, preferably obtainedfrom an AC supply through a current-control device. Variable speedoperation is attained by a control involving a manually set referencevoltage. For any speed setting, constant speed is maintained bydiverting variable amounts of current from the motor circuit throughthyristors, in which the firing times are controlled by the energynecessary to be supplied to the motor to maintain the speed under theexisting load conditions. Continuity of armature current is maintainedat all settings and under all-load conditions.

A dynamic braking system is provided to dissipate the kinetic energy ofthe armature and cause rapid deceleration from a high to a lower setspeed.

BRIEF DESCRIPTION OF THE DRAWINGS In the accompanying drawings,

FIG. I is a diagram of a part of the preferred embodiment of theinvention;

FIG. 2 is a block diagram to illustrate the speed control; I

FIG. 3 is a diagram showing the preferred speed control circuitry;

FIG. 4 is a diagram of the phase control for the thyristors;

FIG. 5 is a vector diagram for the control circuit of FIG. 4;

FIG. 6 is a diagram of the preferred means for obtaining variableresistance for phase control;

FIG. 7 is a diagram of the circuit for controlling the firing of thethyristors;

FIG. 8 is a timing diagram;

FIGS. 9 and 10 are diagrams of a dynamic braking system; and

FIG. 11 is a diagram of a modified system.

GENERAL DESCRIPTION As shown in FIG. 1 the AC supply lines are at 20,21. One of the lines leads through an inductive reactor or a choke 22 toa combined diverter 24 and rectifier 26, which in turn is connected tothe line 21.

The rectifier 26 may be a standard diode bridge comprising, as shown inFIG. 1, four diodes 28, 30, 32 and 34. The diverter comprises twothyristors 36 and 38 preferably of the solid state controlled rectifiertype,

here shown as silicon controlled rectifiers which will be hereinafterdesignated as SCRs.

A thyristor has the property that it will start to conduct current onlyin one direction and then only when the gate is turned ON and willthereafter continue to conduct (even though the gate is turned OFF)until the current cuts off.

The input, as is usual in bridge rectifiers, is connected betweendiagonally opposed junctions designated a and b, namely, at the junctionbetween diodes 28 and 34 and at the junction between diodes 30 and 32,respectively, while the output is taken from the other two diagonalcorners c and d.

The SCRs 36 and 38 are connected across the diodes 32 and 34respectively but each is pointed in the direction opposite to itsparallel diode. Therefore, when the SCRs are in the conducting mode adirect short-circuit path from a to b is established through SCR 38 anddiode 32 in series, and through SCR 36 and diode 34 in series from b toa.

The output junctions connect by leads 40 and 42 with the armature 44 ofa separately excited DC motor 46. A capacitor 50 is connected across theoutput leads 40 and 42 to provide a path for maintenance of armaturecurrent during such portions of the cycle as the supply current passesthrough the diverter SCRs 36 and 38.

The field winding 52 of the motor is separately excited from anysuitable source, as for example a battery or by rectified AC from thelines.

The choke 22 operates as a current control device to maintain aconstant, or nearly constant, current input to the bridge over wideranges of load on the motor. The current supplied to the motor armatureis then varied by diverting more or less of the source current throughthe SCRs 36 and 38. The current paths through the bridge-diverter systemwill now be described.

Assume first that the SCR gates are continuously OFF. The thyristorsconstitute practically infinite impedances across two arms of thebridge, and the bridge therefore acts as if the thyristors were notpresent. Thus on a positive half-cycle current passes through the lead20 and choke 22 to junction a of the bridge, diode 28, lead 40, armature44, lead 42, diode 32 to junction b and its connection to line 21. On anegative half-cycle a similar circuit is traced from b to a but throughdiodes 30 and 34 so that only positive current flows through thearmature, as in normal rectifier bridge operation.

If the thyristors 36 and 38 are continuously turned on they continuouslyshort circuit the constant current source. On a positive half-wavecurrent flows from junction a through SCR 3s and diode 32 to b, while Onthe negative half-wave current flows from b through SCR 36 and diode 34to a. Thus a continuous shortcircuit exists between the input terminals0 and b of the bridge, and no current goes to the armature.

By c5riiiectiontobe"describedpresently, the SCR gates may be turned onat intermediate points of the cycle through a phase-control network tovary the amount of current diverted from the constant current source.Thus on a positive half-cycle current will go through the armature byway of diodes 28 and 32 during the first portion of the cycle when theSCR 38 is turned off and will be diverted from the armature for theremainder of the half-cycle after SCR 38 is turned on; in this latterpart of the half-cycle short-circuit current runs through SCR 38 anddiode 32 in series as above described. When the line current reverses.SCR 38 cuts off; then for the first part of the negative half cyclecurrent flows through the armature through diodes 30 and 34, while forthe last part of the cycle when SCR 36 is turned on the current isdiverted through SCR 36 and diode 34. Therefore by proper phase controlthe SCRs may be turned on at appropriate times in the successive halfcycles to control the supply of current to the armature.

The current control choke 22 performs the function of forming a constantcurrent source to the bridge. The total impedance of the system, asreferred to the AC side, is made up of the reactance of which theinductive reactance X of the choke 22 is the major part, and aneffective resistance R which takes account of the speed and loadconditions of the motor as well as the diversion of current through theSCRs. Thus with total diversion R is nearly zero, while with less thantotal diversion, R times the current squared represents the motor powerplus the losses in the system. The reactance of the choke can be made ofa value comparable to or larger than the maximum value of R and becauseof the quadrature relationship between R, and X, the percentage changeof current will be relatively small over the whole range of R from zeroto its maximum value. Therefore it may be considered that asubstantially constant current source exists at the input junctionterminals a and b of the bridge-diverter circuitry.

Current Diversion Control As heretofore noted the amount of currentdiverted through the diverter circuitry is controlled by timing thegating of the thyristors in their conducting halfcycles, preferably by aphase control of generally wellknown type. A block diagram of thecontrol system appears in FIG. 2. The phase control according to theinvention is carried out by a feedback loop 53-responsive to a speedsetting and to the actual speed of the motor, as indicated by the block54. The speed setting is a manual setting for desired speed, by which areference voltage is generated, and this reference voltage is comparedto a voltage proportional to the motor speed, this speed voltagebeingconveniently generated by a tachometer 48. (Alternatively the speedvoltage may be the counter EMF of the armature measured as the actualterminal voltage of the armature minus a voltage introduced in thecircuit to compensate for the IR drop in the armature as will bedescribed later.)

As shown in FIG. 3 the reference voltage is preferably generated from anAC source and a transformer 55 having a center-tapped secondary leadingthrough a full wave rectifier 56 to a filter capacitor 58 and resistor60. Across the filter is connected a Zener diode 62 for voltageregulation, whereby a substantially constant DC reference voltageappears at the Zener terminals e and f. If desired, there may besubstituted for these elements a battery connected between the terminalse and f The reference voltage at the termir ials e andfmay beconveniently about volts. A cOnnection is made from the positiveterminal e through a variable resistor 64, to a potentiometer 66 andthence to the negative terminalf. The slider 68 of the potentiometer atvoltage V above the negative terminal f is a manual settingcorresponding to a desired speed of the motor. The variable resistor 64is a permanently set or factory adjustment fo fix the maximum value ofV, corresponding to the maximum desired motor speed.

Also connected across the Zener terminals are a fixed resistor 70 and apotentiometer 72, of which the slider also constitutes a factoryadjustment," whereby the voltage V,, of the slider above the negativeterminal can be varied from zero to approximately onethird of theconstant reference voltage. The voltage V corresponds to the minimummotor speed.

There is a potential difference between the slider 68 and the slider 75,the former being normally at the higher potential. The potentialdifference between the two sliders is V,, V and this potential is calledthe reference voltage V This reference voltage opposes a voltageproportional to the actual speed of the motor, which voltage may beobtained from a tachometer 48 driven by the motor. (In a modified formof the invention to be described later, the speed voltage may be takenfrom the armature terminals, with compensation for the voltage drop inthe armature resistance.

As shown in FIG. 3 a connection is made from the slider 68 throughresistors 78 and 80 and the tachometer terminals to the slider 75. Thiscircuit carries a current designated the error current lg, and thevoltage across the resistor 78 is an error voltage E,,. This errorvoltage is e 1a/ 1s R80) (VT a) where the resistance values of theresistors 78 and 80 are indicated by R with appropriate subscripts, V isthe reference voltage as above defined and V is the tachometer outputvoltage.

The error voltage E is therefore variable over a range dependent on thetotal speed range. The system parameters are chosen so that thereference voltage V is always less than the speed voltage V under stableoperating conditions, and the error current 1 then flows in thedirection indicated by the arrow I A certain value of E corresponds tothe condition of the actual speed being equal to the set speed. If E ishigher or lower than that value, the diverters are fired at earlier orlater times in their half-cycles, to decrease or increase the currentsupplied to the motor.

The manner in which the error voltage is used to control the gates ofthe SCRs to time the firing thereof is by a phase control circuit, whichin general principle is of the usual resistance-capacitance type/In FIG.4, a variable resistor is shown diagrammatically at 82 with terminals gand h. The resistor and a capacitor 84 are connected in series acrossthe secondary of a transformer 86. Between g at the junction of theresistorcapacitor and a center tap k of the secondary is a circuitcomprising a resistor 88 and the primary of a transformer 90, acrosswhich is a capacitor 92. The secondaries of the transformer 90 are usedto control the transmission of firing pulses to the gates of the SCR's,as will presently appear. t

The vector diagram for the circuit of FIG. 4 appears in FIG. 5. Sincethe capacitor and variable resistor carry substantially the same current(the current through the g-k' path being limited by the resistor 88),the voltages V and V across the resistor and capacitor are in quadratureand the locus of the ends of their vectors is a semicircle, whereby thevoltage between g and k is of constant magnitude but of varying phase.As the resistance of the resistor 82 is increased the phase of V isretarded.

According to the invention the resistor 82 is varied in accordance withthe error voltage. The lower error voltage, the higher will be theresistance, and the more will the phase of V be retarded, so thatdiversion by the SCRs will occur at later instants.

Although any suitable arrangement for varying the resistor 82 inaccordance with the error voltage may be used, the preferred form of thephase-control resistor is obtained by use of a transistor and fourdiodes connected as shown in FIG. 6. The four diodes designated 102,104, 106 and 108 are connected as a bridge, the input terminals of whichare the terminals g and h of 'the phase-control resistor. The otherjunctions of the minals of the resistor 82 shown in FIG. 4. Forcompari-.

son with FIG. 3, the resistor 80, the tachometer terminals and the errorcurrent l are shown in FIG. 6.

By the arrangement of FIG. 6, the base-emitter voltage of the transistoris the error voltage E.,. The effective resistance of the circuitbetween the terminals g and h is therefore governed by the controlvoltage on the transistor. The higher the error voltage the moretransistor current will flow and hence the lower the effectiveresistance of the path through the transistor will be. The diodes allowa unidirectional current through the transistor with an alternatingvoltage between the terminals g and h. On one half-cycle current flowsfrom g to h through diode 102, transistor 114, resistor 116 and diode106, and on the next half-cycle, the flow is from h to g through diodes108 and 104.

Noting that changes in current through the transistor 114 result inchanges of effective resistance between the terminals 3 and h, it willbe seen from FIG. 4 that these changes result in changes of phase of thevoltage across the primary of transformer 90. The phase of the voltageacross the primary-of transformer 90 may be used in any suitable orwell-known manner to control the firing angle of the diverter SCRs.However, for purposes of the present invention, a special phasecontrolcircuit is preferably used, as shown in FIG. 7.

The primary 90 of the phase control transformer, previously described inconnection with FIG. 4, is shown in FIG. 7. A secondary 118 oftransformer 90 with a capacitor 1 19 across it connects through a diode120 to a parallel resistor 1'21 and a series capacitor 122 which appliescontrol voltage to the base of a transistor 124.

A diode 141 is connected between the emitter and base of the transistor124. At the other side of the figure is shown another secondary 126 ofthe transformer 90 likewise arranged through a similar circuit to applya control voltage to the base of a transistor 128. These transistorcircuits are powered by a full-wave rectifier circuit including atransformer 130 connected to the line and having a center-tappedsecondary leading through diodes 132 and 134 to a lead 135 connected tothe collector terminals of the transistors 124 and 128, whichtransistors are connected through resistors 136 and 138 respectively tothe center-tap lead 139. A filter capacitor 140 is connected between thepositive lead 135 and the center-tap lead 13?. The circuitry associatedwith the transistor 128 is identical with that for the transistor 124,as shown in FIG. 7, and the detailed description is not repeated.

The center-tap lead 139 has a terminal 142 which is connected to thejunction cl of the cathodes of the diverter SCRs 36 and 38 of FIG. 1.

In FIG. 7 the junction of the emitter of transistor 124 and the resistor136 is connected through a resistor 146 to a terminal 147, and thecorresponding junction for transistor 128 is connected through aresistor 148 with a terminal 149. The terminals 147 and 149 areconnected to the gates of the diverter SCRs 36 and 38 respectively,whereby the voltages of these terminals-with respect to the terminal 142constitute the gate-tocathode voltages of the respective SCRs. The meansby which gating pulses are transmitted at proper times to the diverterSCRs are described as follows:

On a rising part of the sine wave of the voltage of the secondary 1 18,applied across capacitor 1 19, the diode 120' conducts and turns thetransistor 124 ON. The

transistor conducts current in saturation until about the peak of thesine wave, so that the voltage across resistor 136 is a flat-toppedpulse about long. As the voltage across the secondary 118 declines fromits peak the capacitor 122 dischargesthrough resistor 121' and diode141, turning off transistor 124 and ending the pulse. The capacitor 122absorbs the difference between the secondary voltage and thebase-emitter voltage of transistor 124. A similar action occurs in thecircuit associated with transistor 128.

The timing diagram for the phase control circuit is presented in FIG. 8.The current through the choke 22 is substantially constant as heretoforeexplained. In the first half-cycle shown in the diagram conductionoccurs through diodes 28 and 32 and this conduction constitutes currentthrough the armature in parallel with the capacitor 50, the SCRs beingthen turned off. At some phase angle a a pulse is applied from thecontrol circuit to the gate of SCR 38 thereby causing short-circuit ordiverter current to flow through SCR 38 and diode 32. Current continuesthrough this diverter circuit until the total current passes throughzero, at which time the SCR 38 is turned off. In the ensuing negativehalf-cycle current flows to the armature through diodes 30 and 34 up tothe same phase angle a at'which time diverter current flows through SCR36 and diode 34 for the remainder of the negative half-cycle.

As heretofore explained, the error current has a value proportional tothe difference between the speed voltage V and the reference voltage VIf the motor has been running under a constant load and at a constantspeed, and if the load is then increased, the motor will momentarilyslow down,.decreasing the speed voltage V andtherefore decreasing theerror current and error voltage. The transistor circuit between g-h ofFIG. 6 then acts like a higher resistance; or stated in another way, theresistance of the variable resistor 82 is increased, and hence the phaseof V,,, is retarded. This causes the diverter SCRs 36 and 38 to firelater in their respective half-cycles, thereby causing less current tobe diverted, so that the motor then receives more current and the motorspeed is restored.

Similar conditions exist if the speed setting is increased. The increasein reference voltage V causes a momentary decrease in error voltage, andthe SCRs fire later in their half-cycles, thereby diverting lesscurrent, so that the motor current increases to bring the actual motorspeed up to the set speed.

Dynamic Braking If the motor is operating at a high speed and it isdesired to change to a lower speed, this is done by setting the control68 to a lower value, thus decreasing the reference voltage and causingan increase in the error voltage V,,. The conditions in going from ahigh speed to a lower speed are, however, different from the conditionsin going from a low to a higher speed. The speed will drop only at arate determined by the dissipation of the kinetic energy of thearmature. In the system thus far described the dissipation of kineticenergy at light loads is largely by friction and windage.

According to the invention provision is made for dissipating energy bydynamic braking in order to cause an acceptably fast deceleration. Asshown in FIG. 9, the error voltage E,,, which is across resistor 78, isapplied to the base of a transistor 150 through a resistor 152. Theemitter of the transistor 150 is biased by a voltage V appearing acrossa resistor 154, which car ries the emitter current of a transistor 155.The transistors are energized from a constant voltage source, indi catedby a Zener diode 156. A potentiometer 158 is connected across the sourceto set the bias voltage on the transistor 155. The bias voltage V ishigher than the highest normal level of the error voltage E so that thetransistor 150 is normally OFF. However, when the reference voltage V issuddenly reduced, as it is when the manual control is set for a lowerspeed, the error voltage E, becomes large and overcomes the bias voltageV so that the transistor 150 is turned on. The current through thetransistor energizes a relay 158, which has contacts 160 to close atrigger circuit to the gate of a dynamic braking SCR 162 (FIG. 10). Acurrent I,, then flows in a circuit as follows: from the positiveterminal of the armature through a dynamic braking resistor 164, SCR162, and thence by a lead 166 to the junction a of the bridge rectifier26. The circuit is completed from point a to the negative armatureterminal in a manner to be presently described. Parts of FIG. 1 arerepeated in FIG. 10 to show the complete dynamic braking circuit.

Provision is made to cut off the current through the dynamic braking SCR162 when the error voltage falls to a level low enough to turn off thetriggering voltage to its gate, since otherwise the SCR would remain inthe l conducting mode. At the time the SCR is turned on at the beginningof the dynamic braking period, the error voltage is at a level toindependently force the diverter SCRs 36 and 38 to their full ONcondition in which they conduct over the full cycle, that is, the anglea is then zero. During the dynamic braking operation the current I,,returns to the negative armature terminal from 'junction a eitherthrough SCR 38 or through diode 34, as follows: If the dynamic brakingcurrent is large (i.e., greater than the diverter current), I,, canreturn through SCR 38 over the full cycle, since the SCR will not'turnoff at any time in the cycle. On the other hand, when the current 1,, issmall, the SCR 38 may cut off at some point in the negative half-cycle.However, during such cut-off, conduction of I will occur, in effect,through diode 34 to junction d and thence to the negative armatureterminal. (Actually at this time diverter current flows from d to athrough the diode 34, and the dynamic braking current may be consideredas flowing opposite to the diverter current; stated in another way, thetotal current through 34 is the diverted current from SCR 36 minus thedynamic braking current I When the motor speed declines toward the setspeed, the current through the transistor falls to a level at which therelay 158 is deenergized, thereby cutting off the gate voltage of thedynamic braking SCR 162, which nevertheless continues to conduct so longas I finds a path to the negative armature terminal either through SCR38 or diode 34. As the speed continues to diminish toward the set speed,the error voltage continues to fall until it causes the angle a toassume a non-zero value. At the same time, the current 1,, has beendiminishing because of the slowing-down of the armature, and thereforeat some time as the setspeed is approached, the SCR 38 becomesmomentariy nonconducting during the first part of the positivehalfcycle. This momentary break of the path causes the dynamic brakingSCR 162 to commutate itself OFF, thus terminating the dynamic brakingoperation, and allowing stable operation under the new speed and loadconditions.

Modified Error Measurement Instead of using a tachometer for generatinga voltage proportional to the speed as in FIG. 3, it is possible toutilize the counter EMF of the armature. The voltage at the armatureterminals may be introduced into the error circuit in place of VHowever, the terminal voltage will be higher than the counter EMF by thel R drop where 1,, is the armature current and R is the armatureresistance.

To compensate for the I R drop the circuit in FIG. 11 may be used inplace of that of FIG. 3. A compensating resistor 164 is in series withthe armature. A potentiometer 167 is connected across the resistor 164,and the slider thereof is connected with the resistor 80. The voltagebetween the slider and the negative end of the compensating resistor issubstantially proportional to the IR drop in the resistor 164. Connectedacross the armature is a resistor 168 in series with a resistor 170, thelatter being connected to the positive end of the potentiometer wherebythe potential difference across 170 is Rue/R R times the terminalvoltage V The voltage across 164 is small so that V approximates thearmature terminal voltage V,,. By proper selection of the ratio of theresistances and the setting of the potentiometer slider, it is possibleto compensate for the voltage drop in the armature resistance, bysubtracting the voltage across the positive end and slider ofpotentiometer 167 from the voltage across R so that the error voltage Eacross resistor 78 correctly represents the difference between a speedvoltage V and a reference voltage V but with the speed voltage derivedfrom the counter EMF of the motor.

Motor Characteristics The speed torque characteristics of the motor withno diversion are standard for a separately excited motor, that is, withdroop in the speed as the torque is increased. Constant speed operationis obtained with current diversion. For any given speed setting at thevoltage divider 66 the diverted current will be such that at any loadthe armature current will be maintained at a value on one of thespeed-torque characteristic curves so that the speed will remainconstant.

SUMMARY OF- OPERATION The current flowing into the bridge-divertercircuit is of substantially. constant amplitude. Considering the firstpositive half-cycle, the diverter SCR 38 is turned off at the beginningof the half-cycle and the total rectified current is available for themotor circuit. At a time represented by the phase angle a, the SCR 38 isturned on by the transmission of a phased pulse to the gate, and all ofthe current is diverted for the remainder of the half-cycle. At the endof the positive half-cycle, the SCR 38 turns off by reason of thereversal of current. In the ensuing negative half-cycle the bridgesupplies rectified current to the armature circuit during the intervalrepresented by the angle a, after which the SCR 36 is turned on and thecurrent is diverted from the armature circuit for the remainder of thehalf-cycle. Therefore the bridge supplies current to the armaturecircuit during the angle a of each half'cycle, as indicated by the solidline graph of FIG. 8.

During the (1r-a) portion of each half-cycle, the bridge does not passcurrent to the armature, but armature current will continue to flowbecause of its inductance and the charge in the capacitor 50. By thismeans continuity of current flow through the armature is established.

The phase angle at which the diverter SCRs fire is determined by acomparison ,of the actual speed with the speed setting. Under lightloads the angle a will be small, and the armature will receive energyfrom the bridge during only a small portion of each half-cycle. Underheavy loads, the angle a will be large, and energy will be deliveredfrom the bridge to the armature during most or all of each half-cycle.

In any case the firing angle is determined by the error voltage, whichis the difference between a voltage proportional to actual speed and theset voltage. The set voltage itself being determined manually for anydesired speed. I

The speed-torque characteristics are those of a typical self-excitedmotor. So long as all conditions are constant the motor operates at aconstant speed and torque. If the load then increases, the speed willtend to fall, and this results in a lower speed voltage and hence alower error voltage, which causes the diverter SCRs to fire at latertimes in their half-cycles, so that increased energy is supplied to themotor to maintain'its speed under'the increase of load. Constant speedoperation therefore involves a shift from one speed-torquecharacteristic to another as the load is varied.

Under conditions in which the load is constant, the error voltage willbe constant, and will in all cases be just sufficient to maintain thespeed at the set value. When an increase in speed is called for by a newsetting of the manual control, the error voltage will decrease, and thiswill call for firing of the diverter SCRs at later points in theircycles, so that more energy will be supplied to the motor to bring itsspeed up to the set value.

Therefore whenever there is an increase of load at a given set speed oran increase of the set speed for a given load, the system automaticallyadjusts itself to increase the energy supplied to the motor. A newequilibrium is established at which the energy supplied to the armatureis just sufficient to maintain the desired speed under the existing loadconditions. The time required for the establishment of the newconditions will be determined by the time constants of the variouscomponents of the system.

A different situation exists, however, when a reduction of suppliedenergy is called for, as a result ofa lowering of the manual set speed.The rotational energy of the armature must then be dissipated, and thisdissipation of energy might require an inordinately long time if itdepended entirely on the friction and windage losses of the motor. Inorder to bring the motor quickly to its new characteristic, the dynamicbraking feature of the invention is utilized. This operates whenever theerror voltage is larger than a certain value. Then the motor is causedto feed energy into the dissipation circuit, wherein the excess energyis quickly dissipated electrically in order to bring the motor into thenew equilibrium condition in which the energy supplied is justsufficient to maintain the load at the lower speed.

A feature of the invention is that under all conditions of operation,continuity of armature current is maintained. Therefore, notwithstandingthe sharp cut-off of current at the diverter circuit, the problems ofpoor commutation, cogging and poor regulation frequently encountered indriving DC motors from rectified current are avoided. Stated in anotherway, the form factor of the armature current is maintained near unity,usually not over about 1.05. The form factor is the ratio of rms toaverage value. A form factor near unity indicates that no discontinuityin the armature current can exist.

-Another feature of the invention lies in the protection afforded byconstant current operation. Thus overloading or stalling of the motor,or short-circuiting of any part of the dc. circuit, cannot result indamage to any part of the system. For the same reason, the SCRs areprotected from shoot-through at all times, regardless of transient oroverload conditions in the system. The system is also protected againstdamage from external conditions, such'as line voltage transients, andthe semiconductors are protected from shootthrough under all conditions.

I claim: I i

l. A variable speed motor system comprising an AC source, a DC motorhaving an armature, a bridge rectifier, an inductor between the sourceand the rectifier to form a substantially constant current source at therectifier, connections from the rectifier to the armature, a capacitoracross the armature, and diverter means comprising thyristors connectedacross arms of the bridge to divert current from said constant currentsource to vary the current passing through the rectifier to be suppliedto the armature.

2. A system as defined in claim 1 in which the thyristors are siliconcontrolled rectifiers having conduction control gates, andmeans forapplying firing potentials to said gates at selected times in successivehalfcycles to control the diversion of current from said constantcurrent source.

3. A system as defined in claim 2, having in addition a referencecircuit, means for introducing into the reference circuit a manuallyselected reference voltage, means for introducing into the referencecircuit a speed voltage dependent on the motor speed, whereby an errorvoltage which is the difference between the speed voltage and thereference voltage exists in the reference circuit, and means controlledby the error voltage for controlling the diverter means.

4. A system as defined in claim 2, in which the means for applyingfiring potentials to the gates comprises a reference circuit, means forintroducing into the reference circuit a manually selected referencevoltage, means for introducing into the reference circuit a speedvoltage dependent on the motor speed, whereby an error voltage which isthe difference between the speed voltage and the reference voltageexists in the reference circuit, and means controlled by the errorvoltage for determining the times at which the firing potentials areapplied to the gates in successive half-cycles.

5. A system as defined in claim 2, in which the means for applyingfiring potentials to the gates comprises a reference circuit, means forintroducing into the reference circuit a manually selected referencevoltage,

means for introducing into the reference circuit a speed voltagedependent on the motor speed, whereby an error voltage which is thedifference between the speed voltage and the reference voltage exists inthe reference circuit, a phase control circuit including a variableresistance for determining the times at which the firing potentials areapplied to the gates, and means for controlling the variable resistancein dependence on the error voltage.

6. A system as defined in claim 3 in which the speed voltage is takenfrom the armature voltage of the motor, and means are provided forcompensating for the IR drop in the armature.

7. A system as defined in claim 6., having in addition a dynamic brakingcircuit, and means for connecting the dynamic braking circuit across thearmature when the error voltage is of a value to call for a reduction ofmotor speed, whereby kinetic energy of the armature is dissipated in thedynamic braking circuit, said dynamic braking circuit including adynamic braking thyristor, means controlled by the error voltage forfiring the dynamic braking thyristor, and means for commutating thedynamic braking thyristor off when the error voltage is in the range ofstable operation, the commutating means including a connection betweenthe dynamic braking circuit and the diverter means to cut off thecurrent through the dynamic braking thyristor when the diverter meansbecomes non-conducting.

mg UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No, 3Dated February Inventor(s) Ai le'n'E. Ottoson It is certified that errorappears in the above-identified patent and that said Letters Patent arehereby corrected as shown below:

Column 5 line 18, cancel "5" and substitute -g Column 12', line 7,cancel "6" and substitute --3.

f Signed and sealed this 13th day of August 197 (SEAL) Attest:

MCCOY M. GIBSON; JR. C. MARSHALL DANN Attesting Officer Commissioner ofPatents 32 3 UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Pa n3,792,330 Dated February 12, 1974 Inventofls) Allen E. Ottoson It iscertified that error appears in the above-identified patent and thatsaid Letters Patent are hereby corrected as shown below:

Column 5,' line 18, cancel "g" and substitute q Column 12', line 7,cancel "6" and substitute --3--.

Signed and sealed this 13th day of August 197 (SEAL) Attest:

MCCOY M. GIBSON, JR. C. MARSHALL DANN Attesting Officer Commissioner ofPatents

1. A variable speed motor system comprising an AC source, a DC motorhaving an armature, a bridge rectifier, an inductor between the sourceand the rectifier to form a substantially constant current source at therectifier, connections from the rectifier to the armature, a capacitoracross the armature, and diverter means comprising thyristors connectedacross arms of the bridge to divert current from said constant currentsource to vary the current passing through the rectifier to be suppliedto the armature.
 2. A system as defined in claim 1 in which thethyristors are silicon controlled rectifiers having conduction controlgates, and means for applying firing potentials to said gates atselected times in successive half-cycles to control the diversion ofcurrent from said constant current source.
 3. A system as defined inclaim 2, having in addition a reference circuit, means for introducinginto the reference circuit a manually sElected reference voltage, meansfor introducing into the reference circuit a speed voltage dependent onthe motor speed, whereby an error voltage which is the differencebetween the speed voltage and the reference voltage exists in thereference circuit, and means controlled by the error voltage forcontrolling the diverter means.
 4. A system as defined in claim 2, inwhich the means for applying firing potentials to the gates comprises areference circuit, means for introducing into the reference circuit amanually selected reference voltage, means for introducing into thereference circuit a speed voltage dependent on the motor speed, wherebyan error voltage which is the difference between the speed voltage andthe reference voltage exists in the reference circuit, and meanscontrolled by the error voltage for determining the times at which thefiring potentials are applied to the gates in successive half-cycles. 5.A system as defined in claim 2, in which the means for applying firingpotentials to the gates comprises a reference circuit, means forintroducing into the reference circuit a manually selected referencevoltage, means for introducing into the reference circuit a speedvoltage dependent on the motor speed, whereby an error voltage which isthe difference between the speed voltage and the reference voltageexists in the reference circuit, a phase control circuit including avariable resistance for determining the times at which the firingpotentials are applied to the gates, and means for controlling thevariable resistance in dependence on the error voltage.
 6. A system asdefined in claim 3 in which the speed voltage is taken from the armaturevoltage of the motor, and means are provided for compensating for the IRdrop in the armature.
 7. A system as defined in claim 6, having inaddition a dynamic braking circuit, and means for connecting the dynamicbraking circuit across the armature when the error voltage is of a valueto call for a reduction of motor speed, whereby kinetic energy of thearmature is dissipated in the dynamic braking circuit, said dynamicbraking circuit including a dynamic braking thyristor, means controlledby the error voltage for firing the dynamic braking thyristor, and meansfor commutating the dynamic braking thyristor off when the error voltageis in the range of stable operation, the commutating means including aconnection between the dynamic braking circuit and the diverter means tocut off the current through the dynamic braking thyristor when thediverter means becomes non-conducting.